Calf rennet, obtained from the fourth stomach of unweaned calves, has traditionally been used as the coagulant for milk in the production of cheese. More recently an enzyme producing during the fermentation of certain fungi has been found to be a suitable replacement for calf rennet, the supply of which is limited by the availability of calf stomachs.
While the milk clotting enzyme from fungi (typically referred to as microbial rennet) is quite suitable for making cheese, it has a higher degree of thermal stability than calf rennet, which property is disadvantageous because the rennet ends up in the whey during the cheese making process resulting in proteolosis of the whey protein. Calf rennet does not present this problem because it is deactivated at ordinary pasturization temperatures. Cornelius reports in U.S. Pat. No. 4,348,482 that the thermal stability of microbial rennet can be decreased without substantially reducing its milk clotting activity by contacting an aqueous solution thereof with a methionine-oxidizing agent. This process, has achieved significant acceptance in the marketplace, especially with rennet from the species Rhizomucor miehei (formerly Mucor miehei). The taxonomy of the genus Mucor was revised and both Mucor pusillus and Mucor miehei were reclassified into a new genus Rhizomucor because they were determined to be sufficiently different from other members of the genus Mucor to justify such reclassification.
Cornelius also reports in U.S. Pat. No. 4,362,818 that the milk coagulating activity of the microbial enzyme obtained from R. pusillus can be increased by acylating the enzyme with selected acid anhydrides, including maleic anhydride. This acylation typically results in an increase of about 50% in the enzyme's activity.
Higashi et al report in U.S. Pat. No. 4,530,906 that the coagulating activity of microbial rennet from R. pusillus can be increased by treating it with succinic anhydride and have reported a low proteolytic activity/milk coagulating activity for the treated enzyme. They also report the method of improving microbial rennet from Mucor pusillus by acylating with a dicarboxylic acid anhydride and then oxidizing with an oxidizing agent.
The R. pusillus rennet which has been treated in the manner described above has obvious advantages as compared to the untreated material. However, it has been observed that its stability in aqueous solution is less than that of the untreated material. For example studies have indicated that the treated material lost 30% of its activity after 3 months at 40.degree. C. and pH 5.3 whereas the untreated material lost only 5% activity. Stabilization of modified rennet from R. pusillus is desirable because such extensive loses of activity which occur even under refrigerated conditions, are unacceptable to cheese manufacturers since lost enzyme activity means increased cost and also increases uncertainty over the quantity of coagulant to add to the cheese milk to obtain a satisfactory set time. A considerable body of literature exists on the topic of enzyme stabilization and has been reviewed by Schwimmer in Source Book of Food Enzymology, AVI Publishing Company, Inc. Pp. 101-103, but there is little literature on the stabilization of rennet solutions because these enzymes are sufficiently stable (unmodified) under typical formulation conditions: 15-20% NaCl, 2% Na propionate, 0.5% Na benzoate, pH 5.0-5.5. Solutions of modified rennet from R. pusillus typically have a natural pH in the range of 5.0 to 5.5 which is the pH they stabilize at when the enzyme powder is dissolved in water. The R. miehei rennet solutions typically have an even lower pH (&lt;pH 4.5) which is probably the pH that exists in the solution after the recovery process for the enzyme is completed. The native enzymes are relatively stable under these conditions, the stability problem only becoming apparent when dealing with the thermolabile (oxidized) rennet.
In practice, the alteration of the rennet solution's pH is straightforward. To a rennet solution, optionally containing sodium chloride (15-20% w/v) and other antimicrobial preservatives, such as sodium benzoate or sodium propionate, there is slowly added, with efficient stirring, a solution of an appropriate base (approximately 1M is a convenient concentration) until the desired pH level is reached. A weak base such as ammonium hydroxide is preferred since it will not cause localized increases in the pH which may tend to inactivate some of the enzyme. For this reason, strong bases such as alkali metal hydroxides, if used at all, should be very carefully added to the rennet solution. In addition, the inactivation of enzyme will be minimized if the base is added thereto slowly at room temperature or lower to thereby ensure dissipation of heat generated during the neutralization reaction.
The method of practicing the present invention and the results achieved thereby are further illustrated by the following examples: